A PON system including an optical line terminal; an optical fiber network forming a configuration in which an optical fiber connected to the optical line terminal is split into a plurality of optical fibers by an optical coupler; and optical network units connected to the respective ends of the split optical fibers, is already implemented.
The PON optical line terminal dynamically allocates bandwidth in an upstream direction to the plurality of optical network units in a time-division manner, to prevent interference between upstream signals. An upstream signal communication band which is set for a communication channel is dynamically allocated to each optical network unit, according to the way in which the time division is performed.
Specifically, the optical line terminal receives in advance, from the respective optical network units, bandwidth requests (requests) for the amounts of data to be sent out in the upstream direction, determines upstream bandwidths for the respective optical network units to be allocated in response to the requests, and provides notification (grants) of transmission permitted bandwidths.
Since each grant includes a transmission start time and a transmission permitted length (a value corresponding to a period of time), each optical network unit can send out a predetermined amount of data in the upstream direction during a predetermined period of time specified in the grant (see, for example, Patent Literatures 1 and 2).
Meanwhile, regarding how upstream transmission bandwidth is allocated in response to the bandwidth requests from the plurality of optical network units, there are, for example, decentralized-type DBA (Dynamic Bandwidth Allocation) in which upon arriving of a request from one optical network unit, bandwidth is allocated to the optical network unit whenever necessary; and centralized-type DBA in which bandwidth requests from a plurality of (typically, all) optical network units are collected in a predetermined cycle and bandwidth is allocated in an integrated manner based on the bandwidth requests.
Of them, in the centralized-type DBA, service classes are classified into a low delay class where a maximum value of delay is set; and a normal delay class where such a maximum value is not set, based on one bandwidth allocation cycle (grant cycle) for an upstream direction. Then, by setting the bandwidth allocation cycle for the low delay class to be smaller than that for the normal delay class, compatibility between the effective uses of low delay and bandwidth is achieved (see Patent Literature 2).